XYLOGLUCAN OLIGOSACCHARIDES WITH AT LEAST 2 ALPHA-D-XYLOSE RESIDUES ACT AS ACCEPTOR SUBSTRATES FOR XYLOGLUCAN ENDOTRANSGLYCOSYLASE AND PROMOTE THE DEPOLYMERIZATION OF XYLOGLUCAN

A xyloglucan-derived pentasaccharide, XYl2.Glc3, was shown by viscomet ry to promote the depolymerisation of xyloglucan by enzyme extracts fr om bean (Phaseolus vulgaris L. cv. Canadian Wonder) leaves and pea (Pi sum sativum L. cv. Alaska) stems. Xyl2.Glc3 was also shown by a radioc hemical assay to act as an acceptor substrate for xyloglucan endotrans glycosylase activity (XET; EC 2.4.1.-) present in the same extracts. I n both these assays, a heptasaccharide (XYl3.Glc4) was more effective than XY12.GlC3 whereas two isomeric tetrasaccharides (Xyl1.Glc3) were essentially ineffective. The agreement in the structural requirements of the two assays suggests that they share a common basis; we therefor e propose that the oligosaccharide-sensitive enzyme that depolymerises xyloglucan is XET rather than cellulase (EC 3.2.1.4). In the viscomet ric assay, the penta- and heptasaccharides would, according to our int erpretation, compete with high molecular weight xyloglucan molecules a s acceptor substrates for XET, leading to a decrease in the weight-ave rage molecular weight of the xyloglucan and, therefore, to a decrease in viscosity. Our results indicate that oligosaccharides have to posse ss two alpha-D-xylose residues in order to act as acceptor substrates for XET. The non-reducing end of a high-molecular weight xyloglucan ca n also act as an acceptor substrate. Therefore, it is likely that exo- hydrolysis by alpha-D-xylosidase would destroy the ability of a polysa ccharide to act as an acceptor, even though alpha-D-xylosidase may rem ove only a single xylose residue from each polysaccharide molecule.